Respiration is an important parameter that gives an indication of the physiological state of a person, e.g., whether the person is in a relaxed state or sleeping. Sleep and relaxation parameters such as respiration can be measured in different ways, and sleep laboratories often make use of obtrusive sensors such as EEG/PSG or other on-body sensors such as respiratory belts. Unobtrusive measuring of sleep parameters is more convenient for the sleeper, and unobtrusive measuring is preferred over existing obtrusive measurement techniques for home, medical and lifestyle solutions.
A living body information detection and display method is disclosed in US 2008/0312516 A1, together with a sleeping posture and position apparatus. This is used to correctly and suitably capture an abnormal condition in a living body while the living body is sleeping/lying on a bed or on the apparatus. Living body information and sleeping posture/position are captured by pressure sensors placed under a sleeper. The living body information may include, for example, respiratory information, body movement information, and sleeping posture.
Malakuti, K. reports a data acquisition system for monitoring sleep, wherein said data acquisition system uses 144 pressure sensors embedded in a bed sheet (Intelligent Bed Sensor) for measuring the pressure that the patient's body exerts on the bed. A video camera and a microphone recorded additional information to be used for evaluating the Intelligent Bed Sensor and are not part of the data acquisition system (Malakuti, I. Towards an Intelligent Bed Sensor: Non-intrusive Monitoring of Sleep Disturbances via Computer Vision Techniques; 2008, Thesis, University of Victoria).
To detect respiration unobtrusively, a set of small microphones can be used that can be positioned on the side of a person's bed, close to the persons's head. For example, Sériè, F. and Marc, I. reported recording of snoring by two microphones which were placed symmetrically on each side of a bed, 70 cm above the bed surface and 85 cm apart, and angled to point directly toward the centre of the bed at the normal head position. The signal was preamplified, mixed, equalised and analysed with a spectrum analyser to get the snoring sound pressure level. To take into account the acoustic characteristics of the recording system, a calibration of the sound signal preceded the sleep studies. The sensitivity of the equaliser was adjusted to give a Pink Noise an intensity of 77 dB SPL. Following this procedure the equalised signal of the breathing noise was analysed by the spectrum analyser and transferred to a computer for interpretation (Sériè, F. and Marc, I., Thorax 1994; 49:562-566).
However, breathing produces a relatively weak sound signal compared to snoring. Moreover, other sounds than just breathing may be present in a bedroom or relaxation area, e.g., from ventilation, from movements of the person him/herself, noises or breathing of another person, and the like. These other sounds can be regarded as noise that is detected on top of the “desired” respiration signal, and renders measuring and analysis of the breathing more difficult.
Using two microphones at the same time, wherein said microphones are arranged on opposite sides of the person's head, may be useful for proper detection of the person's respiration. For example, the two signals from said microphones may be combined. However, this method may be improved upon by taking into account information on the person's head position.